1. Field of the Invention
The present invention relates to a method and circuit arrangement for aligning signals, which may be used in any signal processing system.
2. Description of the Invention
Accurate signal alignment functions for aligning two signals are required in many kinds of applications. As one example, digital pre-distortion based linearization circuits require accurate signal alignment, for the following reasons.
The development of digital linearization circuits capable of responding to both fast and slow variations in power amplifier non-linear characteristics is an important step toward cost-effective bandwidth-efficient communications. Two kinds of distortion are to be considered, i.e. amplitude and phase distortions. Real amplifiers have a maximum output power, i.e. saturation level, and an input-output power relationship which will depart from a linear relationship as the output power approaches the saturation level. This is referred to as AM/AM distortion. Similarly, a phase shift depending on the power level will also occur, referred to as AM/PM distortion. The final effect of AM/AM and AM/PM distortion is the generation of unwanted spectral energy both in-band (within the transmission channel) and out-of-band (outside the transmission channel). The in-band energy will cause distortion of the transmitted signal and the out-of-band energy will cause adjacent channel interference (ACI).
In order to improve the efficiency and reduce the size and cost of broadcast transmitters, the linearity of the power amplifier or other concerned circuit arrangements must be increased. This can be achieved by using pre-distortion techniques. A pre-distorter is a device which generates a distortion that compensates for the power amplifier distortion. The result of pre-distortion is that the power amplifier can operate at higher power with the same level of distortion or at the same power with lower distortion. Theoretically, signal pre-distortion can be realized at radio frequency (RF), at intermediate frequency (IF) or at base-band.
In its simplest analog implementation, a practical pre-distorter can be a network of resistors and non-linear elements such as diodes or transistors. However, due to the fact that amplifier characteristics are not constant, but vary with time, frequency, power level, supply voltage and environmental conditions, only moderate performance can be achieved by simple analog pre-distortion techniques. Better results have been achieved by adaptive pre-distortion schemes where the compensating non-linearity is implemented in digital signal processing (DSP) hardware. This approach provides the advantages that the pre-distorter is completely digital, resulting in a very stable product that does not require alignment or tuning in production and that is totally insensitive to environmental variations, and is capable of being augmented with adaptive circuitry that does not require training or periodic sequences to become a fully automatic corrector.
FIG. 1 shows a schematic block diagram of an adaptive pre-distortion scheme. A digital adaptive pre-distorter 10 is located at the input terminal of a power amplifier 20, wherein the output signal of the power amplifier 20 is fed back to a signal processing control unit 30 which also receives the input signal supplied to the pre-distorter 10. It is noted that the signals supplied to, output and processed by the pre-distorter 10 and the digital signal processing unit 30 are digital signals, while the signal amplified by the power amplifier 20 is an analog signal. Therefore, respective digital-to-analog and analog-to-digital circuits have to be provided, which are not shown in the schematic diagram of FIG. 1.
In order for a digital adaptive pre-distortion (DAPD) system to work effectively, two things are required. First, accurate time-alignment of the amplifier input and output signals in the digital domain has to be achieved, and, second, an accurate amplifier model which can correct for all necessary distortions to the required level must be established. It is the time-alignment of the input and output signals that is the subject of this invention.
By its very nature, however, this requires the alignment of two similar, but not identical signals. The transmitted signal is distorted as it is amplified, so the signal fed back to the alignment algorithm at the signal processing unit 30 after amplification is no longer the same as the original signal output by the pre-distorter 10, due to AM/AM and AM/PM distortion. The degree of this difference will be dependent on the DAPD architecture and the convergence state of the system.